Gas separating membranes from polyimide polymers and a process for using the same

Abstract

The present invention is a class of semi-permeable polyimide membranes and a process for using such membranes to separate one or more components of a gaseous mixture. The membranes are prepared from a polyimide having surface-modifiable functionalities comprising repeating units formed from an aromatic dianhydride and a bridged alkylated bisaniline wherein the bridging group of the diamine possesses restricted, rotatable rigid or low Van Der Waal energy substituents which are orthogonal to the polymer backbone. The membranes formed therefrom are contacted in the presence of an oxygen source with an activating force such as high energy electromagnetic irradiation to effect modification of the polymer surface. The surface-modified polyimide membranes exhibit high gas perm-selectivities and are especially useful in effecting the separation of oxygen and nitrogen from air.

We claim: 
     
       1. A semi-permeable membrane formed of a polyimide homopolymer having surface modifiable functionalities, the polyimide homopolymer comprising repeating units of the general structural formula: ##STR4## wherein each R 1  is independently selected from a hydrogen atom, an alkyl having from 1 to about 8 carbon atoms, aryl or an arylalkylene; each R 2  is independently selected from an alkyl having from 1 to about 8 carbon atoms, perhaloalkyl or an arylalkylene; and   A 1  is a single bond or ##STR5##  wherein R 3  and R 4  are independently selected from a hydrogen atom, CF 3 , CF 2  Cl, an alkyl having from 1 to about 8 carbon atoms, phenyl, 1-arylalkyl, arylalkylene, naphthyl or a halogen; wherein the polyimide homopolymer having surface-modifiable functionalities is contacted in the presence of an oxygen source with a source of high energy electromagnetic irradiation or a free radical source to affect modification of the polymer surface.     
     
     
       2. The semi-permeable membrane according to claim 1 wherein the oxygen source is an atmosphere containing at least 5% oxygen. 
     
     
       3. The semi-permeable membrane according to claim 2 wherein the atmosphere containing at least 5% oxygen is air. 
     
     
       4. The semi-permeable membrane according to claim 1 wherein the source of high energy electromagnetic irradiation is ultraviolet irradiation. 
     
     
       5. The semi-permeable membrane according to claim 4 wherein the ultraviolet irradiation source is focused between about 200 and 300 nm. 
     
     
       6. The semi-permeable membrane according to claim 4 wherein the polyimide is contacted with the ultraviolet irradiation source in the presence of a photosensitizer. 
     
     
       7. The semi-permeable membrane according to claim 1 which has been contacted with a free radical source selected from the group consisting of volatile peresters, peroxides and azo compounds. 
     
     
       8. The semi-permeable membrane according to claim 1 wherein R 1  and R 2  are isopropyl and R 3  and R 4  are hydrogen atoms. 
     
     
       9. The semi-permeable membrane according to claim 1 wherein R 1  and R 2  are methyl, R 3  is CF 3  and R 3  is naphthyl. 
     
     
       10. The semi-permeable membrane according to claim 1 wherein R 1  is methyl, R 2  is isopropyl, R 3  is CF 3  and R 4  is phenyl. 
     
     
       11. A process for separating a gaseous mixture containing two or more components, the process comprising: bringing the gaseous mixture into contact with one side of the semi-permeable membrane according to claim 1 whereby one component selectively permeates through the membrane at a different productivity rate than at least one other component. 
     
     
       12. The process according to claim 11 wherein the gaseous mixture comprises nitrogen and oxygen. 
     
     
       13. A semi-permeable membrane formed of a polyimide homopolymer having surface modifiable functionalities, the polyimide homopolymer comprising repeating units of the general structural formula: ##STR6## wherein each R 1  is independently selected from a hydrogen atom, an alkyl having from 1 to about 4 carbon atoms or aryl; each R 2  is independently selected from an alkyl having from 1 to about 4 carbon atoms or arylalkylene; and   R 3  and R 4  are independently selected from a hydrogen atom, CF 3 , CF 2  Cl, an alkyl having from 1 to about 4 carbon atoms, phenyl, 1-arylalkyl, arylalkylene or naphthyl; wherein the polyimide homopolymer having surface-modifiable functionalities is contacted in the presence of an oxygen source with a source of high energy electromagnetic irradiation or a free radical source to affect modification of the polymer surface.     
     
     
       14. The semi-permeable membrane according to claim 13 wherein the ultraviolet irradiation source is focused between about 200 and 300 nm. 
     
     
       15. The semi-permeable membrane according to claim 14 wherein the oxygen source is an atmosphere containing at least 5 mole % oxygen. 
     
     
       16. The semi-permeable membrane according to claim 15 wherein the oxygen source is air. 
     
     
       17. A process for separating a gaseous mixture containing two or more components, the process comprising: bringing the gaseous mixture into contact with one side of the semi-permeable membrane according to claim 13 whereby one component selectively permeates through the membrane at a different productivity rate than at least one other component. 
     
     
       18. The process according to claim 17 wherein the gaseous mixture comprises nitrogen and oxygen.